Choosing Procedures and Managing Hazards
A concise revision guide to choosing a practical procedure for a reaction: which technique suits which functional group and product, the Quickfit apparatus used for reflux and distillation, and justifying the choice of apparatus, solvent, temperature and reagent excess.
Before You Start
Exam questions describe a preparation and ask you to choose, or to justify, the apparatus and conditions. Every choice follows from the properties of the compounds involved.
- Heating under reflux and simple distillation, with the labelled apparatus, are covered on Preparing and Purifying Organic Liquids.
- The reactions themselves come from the reaction map on Planning a Synthetic Route: this page is about how to carry them out.
- Purifying a solid product is on Preparing and Purifying Organic Solids and purifying a liquid on Purifying Liquids and Checking Purity.
- Quickfit glassware with ground-glass joints was introduced at AS; this page names the parts and when each set-up is used.
Choosing a Procedure: Match the Method to the Reaction
A practical procedure is the set of choices about apparatus, temperature, solvent and quantities. Each choice answers a question about the compounds: how volatile, how flammable, how fast, how stable?
| Situation | Procedure | Reason |
|---|---|---|
| slow reaction with volatile or flammable reagents | heat under reflux | vapour condenses and returns, so nothing escapes during long heating |
| the product is volatile and would react further | distil the product off as it forms | an aldehyde boils below its alcohol, so it leaves before it is oxidised to the acid |
| a liquid product mixed with reagents and by-products | distil after the reaction and collect the right fraction | a pure liquid distils at a steady boiling point |
| exothermic or unstable step | ice bath, add the reagent slowly | keeps the temperature down: nitration of benzene is held below 55 °C to avoid dinitration |
| a flammable liquid must be heated | water bath or electric heating mantle | no naked flame near ethanol, ethoxyethane or a volatile haloalkane |
| a reversible reaction or an expensive reagent | use an excess of the cheaper reagent | pushes the position of equilibrium towards the product and uses up the costly reagent |
| a wet organic liquid | add an anhydrous drying agent, then decant | anhydrous magnesium sulfate or calcium chloride takes up the dissolved water |
Seven situations and the procedure that fits each one. Learn the reason as well as the name.
Exam focus: Name the procedure and give the property that demands it: “heat under reflux because bromoethane is volatile and would escape” scores; “reflux because it is a slow reaction” alone does not.
Quick Check: Pick the Procedure
Each line describes a job in a new preparation. Choose the better procedure.
Solvent, Temperature and Quantities
Three further choices complete the procedure, and each must be justified in the same way.
Solvent
- Water for hydrolysis and for substitution of a haloalkane with aqueous NaOH or KOH.
- Ethanol for KCN with a haloalkane (both dissolve) and for KOH when elimination to the alkene is wanted.
- A mixed solvent, such as ethanol and water, gives a mixture of substitution and elimination products.
Temperature
- Warm or heat under reflux for slow reactions; the higher the temperature, the faster the reaction.
- Keep cool when a side reaction takes over at higher temperatures: nitration above 55 °C gives dinitrobenzene.
- Never a naked flame with a flammable liquid: water bath for anything boiling below 100 °C, electric mantle above.
Quantities
- An excess of one reagent drives a reversible reaction towards the product and makes the other reagent the limiting one for the yield.
- Choose the cheaper, safer or easier-to-remove reagent as the one in excess: excess ethanol in an esterification distils off at the end.
- Keep the scale small when a reagent is hazardous; the yield calculation is the same at any scale.
Remember: Solvent: does everything dissolve, and does it steer the reaction? Temperature: fast enough without side reactions. Quantities: excess of the cheap reagent, the other one limits the yield.
Check: Choosing a Technique
Choose reflux, distillation, an ice bath or a water bath for preparations that are not on this page, and say which reagent goes in excess.
Quickfit Apparatus
Quickfit is a set of glassware with standard ground-glass joints. The pieces push together to make a leak-free assembly for reflux or distillation without corks or bungs.
| Part | What it does |
|---|---|
| pear-shaped or round-bottomed flask | holds the reaction mixture; sized so it is one third to two thirds full |
| Liebig condenser | vertical for reflux, sloping for distillation; water in at the bottom, out at the top |
| still head | sits on the flask in distillation and carries the thermometer and the side arm to the condenser |
| thermometer in a screw-cap adapter | bulb level with the side arm, so it reads the temperature of the vapour that distils |
| receiver adapter | joins the condenser to the receiving flask |
| plastic joint clips | hold each joint together so the apparatus cannot fall apart |
- Add anti-bumping granules before heating so the liquid boils smoothly.
- Never seal the apparatus: the top of a reflux condenser and the receiver stay open to the air, or pressure builds and the glass can shatter.
- Heat with an electric heating mantle or a water bath, never a naked flame, because organic liquids are flammable.
- Check every joint is seated before heating; a leaking joint loses vapour and can release a flammable or harmful gas.
Quickfit set up for heating under reflux and for distillation, with the parts named and the water flow marked.
Exam focus: For a diagram mark: closed condenser jacket, water in at the lower end, no gap between flask and condenser, open top in reflux, thermometer bulb at the side arm in distillation.
Quick Check: Spot the Faults
A student has written down how a Quickfit reflux set-up was assembled, and some details are wrong.
Which Technique for Which Product?
Preparing an organic solid and preparing an organic liquid use the same reaction stage but different purification stages.
| Stage | Organic solid | Organic liquid |
|---|---|---|
| reaction | heat under reflux, or warm in a water bath | heat under reflux, or distil the product as it forms |
| isolation | cool, crystallise, filter under reduced pressure | distil; separate the layers in a separating funnel |
| purification | recrystallise from a suitable solvent | wash, dry with an anhydrous salt, redistil |
| purity check | melting point sharp and matching the data book | boiling point steady and matching the data book |
- The solid route in full, with recrystallisation and the melting point test, is on Preparing and Purifying Organic Solids.
- The liquid route, with the separating funnel and drying, is on Purifying Liquids and Checking Purity.
- Decide first whether the product is a solid or a liquid at room temperature; the purification follows from that.
Remember: Solid: filter, recrystallise, melting point. Liquid: separate, wash, dry, redistil, boiling point.
Working Safely
Every procedure comes with sensible precautions, and a question can ask you to name them for the compounds in use.
- Flammable liquids (ethanol, ethoxyethane, cyclohexene): heat with a water bath or electric mantle, no naked flames.
- Toxic or harmful vapours (bromine, concentrated acids giving fumes, phenylamine): work in a fume cupboard.
- Corrosive reagents (concentrated sulfuric acid, sodium hydroxide, acyl chlorides): goggles and gloves, add slowly.
- Exothermic additions: cool the flask in ice and add the reagent dropwise.
- Volatile products: cool the receiver in ice so the product does not evaporate away.
- Keep the scale small and dispose of organic waste in the labelled container, never down the sink.
Exam focus: Match the precaution to the property: “cyclohexene is flammable, so no naked flames” is the full answer; a list of equipment is not.
Worked Example: Justifying a Procedure for Ethyl Ethanoate
Ethyl ethanoate is made from ethanol and ethanoic acid with a concentrated sulfuric acid catalyst. Every step of the procedure can be justified from the properties of the four liquids involved.
The esterification equilibrium with its conditions, and the boiling point of each liquid: ethanol 78 °C, ethanoic acid 118 °C, ethyl ethanoate 77 °C, water 100 °C.
Question. Justify each stage of the procedure used to prepare a pure sample of ethyl ethanoate.
Step 5. Run off the aqueous layer, dry the ester with anhydrous magnesium sulfate until the liquid is clear, then decant and redistil.
Exam wording: A justification pairs each step with the property it deals with: “distil at 77 °C because ethyl ethanoate is the most volatile component” is the whole mark in one sentence.
Check: Justifying a Procedure
Justify the apparatus, solvent, excess and drying choices for preparations that are not worked above.
Common Exam Points
- Reflux keeps volatile reagents in the flask during long heating; distillation takes the most volatile liquid out, during or after the reaction.
- Distil an aldehyde off as it forms so that it is not oxidised further; reflux to take a primary alcohol all the way to the acid.
- Ice bath for exothermic or unstable steps (nitration below 55 °C); water bath or electric mantle, never a flame, for flammable liquids.
- Use an excess of the cheaper reagent to push a reversible reaction towards the product; the other reagent limits the yield.
- Aqueous solvent for substitution, ethanolic for elimination or for KCN; always say which and why.
- Quickfit joints need no corks or bungs, and a thin smear of grease stops them sticking; clip the joints, add anti-bumping granules, water in at the bottom of the condenser, and never seal the apparatus.
- Solid product: filter under reduced pressure, recrystallise, melting point. Liquid product: separating funnel, dry, redistil, boiling point.
- A justification links each step to a property: volatile, flammable, slow, reversible, unstable or acidic.
FAQs
Use these quick answers to check when to use reflux, distillation or an ice bath, why one reagent is in excess, how Quickfit apparatus is assembled safely and how to justify a whole procedure.
When do I heat under reflux and when do I distil?
Reflux when the reaction is slow and the reagents are volatile: the vapour condenses and runs back, so nothing is lost during long heating. Distil when the product is volatile and must be removed as it forms, such as an aldehyde before it oxidises further.
Why is one reagent used in excess, and which one?
An excess pushes a reversible reaction towards the product and makes sure the other reagent is used up. Choose the cheaper reagent or the one that is easiest to remove afterwards: excess ethanol in making ethyl ethanoate is removed by washing and distillation.
Why must Quickfit apparatus never be sealed during reflux or distillation?
Heating a closed system raises the pressure until the glass shatters. In reflux the top of the condenser stays open; in distillation the receiver is open to the air. Add anti-bumping granules before heating so the liquid boils smoothly, and clip the ground-glass joints together.
What are the parts of a Quickfit distillation set-up?
A pear-shaped or round-bottomed flask holds the mixture, a still head carries the thermometer with its bulb level with the side arm, a Liebig condenser with water entering at the lower end cools the vapour, and a receiver adapter leads to the collecting flask.
How do I justify the procedure for preparing ethyl ethanoate?
Reflux ethanol and ethanoic acid with a little concentrated sulfuric acid (catalyst), using excess ethanol to shift the equilibrium. Distil off the ester, wash with sodium carbonate solution to remove acid, dry with anhydrous calcium chloride, then redistil, collecting at its boiling point, 77 °C.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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